Method and apparatus for controlling blast furnaces



Feb. 4 1958 R. T. HANNA ETAL 2,822,257

METHOD AND APPARATUS FOR CONTROLLING BLAST FURNACES Filed June 21, 19552 Sheets-Sheet l hrs-K 1- THROTTLE l5 ,8 COIVTROL MIXER mm: CONTROL ausrvowus co/vmaL f 26 PRESSURE IND/CA ran PR5 881m CHANGE INDICATOR 40mm mrs P n "70, 0,127?" TEMPERATURE cor/mo;

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RALPH r. HANNA and CARL a. HOGBERG,

their AIM/(76y.

Feb. 4 1958 R. T. HANNA ET AL 2,322,257

METHOD AND APPARATUS FOR CONTROLLING BLAST FURNACES Filed June 21, 19552 Sheets-Sheet 2 HOURS RALPH 7'. HAN/VA 0/70 CARL 6. HOGBERG,

8Y1 domwaa/Oza their Attorney.

United States Patent METHOD AND APPARATUS FOR CONTROLLING BLAST FURNACESRalph T. Hanna, Fairless Hill, and Carl G. Hogberg,

Glenshaw, Pa., assignors to United States Steel Corporatron, acorporation of New Jersey Application June 21, 1955, Serial No. 516,848Claims. (Cl. 75-41) This invention relates to an improved method ofoperating a blast furnace.

Within limits, a blast furnace becomes more eflicient as the volume andtemperature of its hot air blast increase. However, furnace pressure isa function of factors which include this volume and temperature, andexcessive pressure interferes with proper descent of the burden and maycause it to hang and later slip, and thus lead to serious difliculties.Therefore, the maximum blast volume and temperature are limited byextreme pressure conditions that just approach hanging. It is customaryto observe the absolute pressure within a blast furnace and control thevolume and temperature to maintain this pressure at a safe value, butall previous controls with which we are familiar rely exclusively onabsolute pressure values. Such controls commence cutting back the volumeor temperature only after the absolute pressure surpasses a designatedvalue, which must be far enough below the maximum safe value or criticalto avoid difiiculties. The margin between the pressure value at whichthe control operates and the critical value of course represents adirect efliciency loss.

An object of our invention is to provide an improved operating methodwhich enables the blast volume and temperature to be maintained athigher values closer to the critical without risking hanging.

A further object is to provide an improved operating method wherein theblast volume and temperature are controlled in accordance with anadditional phenomenon, namely rate of change of furnace pressure.

A further object is to provide an improved operating method whereineither the blast volume or temperature can be continually variedaccording to a pattern to produce maximum efliciency, that is,periodically one or the other is increased until pressure conditionsapproach the danger point and then is cut back to a safe starting point.

In the drawing:

Figure 1 is a schematic showing of a blast furnace equipped with meansfor controlling the blast volume and temperature in accordance with therate of pressure change; and

Figure 2 is a graph, not to scale, showing the general characteristicsof the temperature and volume of the blast of a furnace equipped asshown in Figure 1 and operated according to our invention.

Our invention is based on our discovery that a rapid rate of pressureincrease usually foretells furnace conditions which cause hanging,irrespective of the absolute pressure. We have observed thatconsiderably higher absolute pressure can be tolerated than previouslyhas been considered possible, provided this high pressure is notpreceded by an unduly rapid rate of pressure increase. Of courseabsolute pressures are not neglected altogether, but the absolutepressure at which the control means cuts back the volume or temperaturecan be considerably higher than that used previously. It is diflicult toascribe definite values at which blast furnace pres- 2,822,257 PatentedFeb. 4, 1958 sures, volumes and temperatures are maintained, since thesevalues involve so many other variables, notably the characteristics ofthe rawmaterials and the furnace design. For example, considerablyhigher values are prevalent in the Birmingham, Alabama district than inthe Pittsburgh, Pennsylvania district. Consequently we are able only tofurnish typical examples of the way in which our control operates, andpoint out that the same principles apply where the absolute values areconsiderably different.

Figure 1 shows a typical blast furnace installation which includes afurnace 10, a plurality of stoves 12, a blower 13, and cold and hotblast lines 14 and 15. The furnace has the, usual bustle pipe 16 andtuyeres 17. In accordance with usual practice, the blower 13 directscold air through the cold blast line, into the stoves, and thencethrough the hot blast line, bustle pipe and tuyeres and into the lowerportion of the furnace. The temperature of the blast entering thefurnace is commonly about'700 to 1600" F., the average being about 1000to 1200 F. The stoves are capable of heating the blast considerablyhigher than the maximum temperature at which the furnace operatessmoothly. Therefore, aportion of the blast goes directly from the coldblast line 14 to the hot blast line 15 via a by-pass 18. An adjustablemixer valve 19 is mounted in this by-pass to control the portion of theblast which takes this route and thus control the temperature of theblast reaching the furnace. The mixing valve has a mechanical control 20for varying its opening. The blower 13 has a throttle control 21 forvarying the blast volume, which commonly is about C. F. M. per squarefoot of hearth area.

Also in accordance with usual practice, temperature and pressureindicators 22 and 23 are connected into the hot blast line 15 to showthe temperature and absolute pressure of the blast entering the furnace.The temperature indicator 22 operates an automatic temperature control24, which in turn operates the mixing valve control 20 to maintain thetemperature at any desired value. The individual indicators and controlsare well known instruments and hence are not shown in detail. In atypical blast furnace having a hearth diameter of 28 feet and lackingour control, the normal operating conditions may be:

The critical pressure which causes hanging may be 24 p. s. i. g., whichwould appear under such conditions as:

blast volume O. F. 96, 000 90, 000 93,000 blast temperature-.. F. 1, 4001, 600 1, 500 blast pressure p. s. i. g... 24 24 24 To insure againsthanging it is necessary to commence cutting back the temperature orvolume whenever the pressure commences to exceed the normal value of 20p. s. i. g. The four pound margin is needed as a safety factor, but ofcourse represents an efficiency loss.

In accordance with our invention, an indicator 25 which shows the rateof pressure change in the hot blast line is connected with the pressureindicator 23. In its simplest form the indicator 25 can be a recorderwhich continuously plots the pressure. The rate of change can bedetermined by observation of the slope of the pressure curve. As long asthe curve is a substantially straight line, there is no appreciable ratechange, but if the curve swerves upwardly, the rate is increasing.Whenever the rate increase becomes excessive, the operator can manuallyadjust either the temperature control 24 or a volume control 26connected to the throttle consubstantially higher than in conventionalpractice.

trol 21 to cut back the temperatureor volume, even though the absolutepressure remains below the value at which these factors would be cutback. Nevertheless weprefer to employ an indicator which'automaticallycuts back these factors in response to excessive rates of pressureincrease. Automatic regulators for this purpose are simple adaptationsof well known commercial instruments. As previously pointed out, we setthe controls so that the absolute pressure at which they operate is Inthe aforementioned furnace we might commence cutting back when thepressure rise reached a rate of 1 p. s. i. g. per minute, althoughabsolute pressure could go as high as 24 p. s. i. g. t I

In operating the furnace we prefer gradually to increase the blastvolume while maintaining the blast temperature substantially constant.Under such conditions the pressure of. course rises. Before hangingoccurs, the rate of pressure rise suddenly commences to increase. Atthis point we cut back the volume or temperature or both to a safe valueand repeat the sequence. Thus we are able to operate the furnace atpressures extremely close to the critical without risking hanging.

As a specific example of our invention, the volume of blast delivered toa furnace was set at 80,000 C. F. M. and the temperature at about 1250F. The blast pressure was about 20 p. s. i. Over a /2 hour period thevolume was gradually increased to about 92,000 C. F. M., as indicated bycurve A in Figure 2. At first there was no noticeable increase in therate of pressure rise even though the absolute pressure was rising.After about 4 hours the rate of pressure rise commenced to increase, asshown by curve B in Figure 2. After 5% hours the blast volume was cutback to about 85,000 C. F. M. and the blast temperature briefly loweredto about 1150 F., as indicated by curves A and C. Again the volume andtemperature were increased. This time the rate of pressure risecommenced to increase sooner, since the starting volume was higher, butotherwise the steps were repeated according to the same pattern.Although the curves show both the volume and temperature factorsdecreased in response to pressure rate increases, it is apparent thatlargely equivalent results can be obtained by decreasing only one ofthese factors.

' When the furnace was operated as just described, no hanging occurredalthough the conditions just approached hanging when the volume andtemperature were cut back. The furnace efliciency was improved becausethe volume and temperature factors were kept above the maxima at whichthe furnace operates smoothly in the absence of our control.

While we have shown and described only a single embodiment of ourinvention, it is apparent that modifications may arise. Therefore, we donot wish to be limited to the disclosure set forth but only by the scopeof the appended claims.

We claim:

1. In the operation of a blast furnace, wherein a preheated air blast isapplied to the lower portion of the furnace and the pressure within thisportion is a function of factors which include volume of the blast andtemperature of the blast, a method of maintaining this pressure atabsolute values close to the critical that causes hanging comprisingcontinuously measuring the rate of pressure change, increasing at leastone of said factors as long as no increase occurs in said rateregardless of increases in the absolute pressure up to the limit whichitself causes hanging and cutting back one of said factors when anincrease is detected in said rate.

2. In the operation of a blast furnace, wherein a pre heated air blastis applied to the lower portion of the furnace and the pressure withinthis portion is a function of factors which include volume of the blastand temperature of the blast, a method of maintaining this pressure atabsolute values close to the critical that causes hanging comprisingperiodically gradually increasing at least one of said factors from astarting point substantially below that which produces a criticalpressure and thereby gradually increasing the absolute pressure,continuously measuring the rate of pressure change as the absolutepressure increases, continuing to increase this factor as long as noincrease occurs in said rate regardless of increases in the absolutepressure up to the limit which itself causes hanging, and cutting backthis factor when an increase is detected in said rate.

3. A method as defined inclaim 2 in which the factor periodicallyincreased and cut back is the volume of the blast.

'4. A method as defined in claim 2 in which the factor periodicallyincreased and cut back is the temperature of the blast.

5. A method as defined in claim2 in which both the volume andtemperature of the blast are periodically raised and cut back.

References Cited in the file of this patent UNITED STATES PATENTS

2. IN THE OPERATION OF A BLAST FURNACE, WHEREIN A PREHEATED AIR BLAST ISAPPLIED TO THE LOWER PORTION OF THE FURNACE AND THE PRESSURE WITHIN THISPORTION IS A FUNCTION OF FACTORS WHICH INCLUDE VOLUME OF THE BLAST ANDTEMPERATURE OF THE BLAST, A METHOD OF MAINTAINING THIS PRESSURE ATABSOLUTE VALUES CLOSE TO THE CRITICAL THAT CAUSES HANGING COMPRISINGPERIODICALLY GRADUALLY INCREASING AT LEAST ONE OF SAID FACTORS FROM ASTARTING POINT SUBSTANTIALLY BELOW THAT WHICH PRODUCES A CRITICALPRESSURE AND THEREBY GRADUALLY INCREASING THE ABSOLUTE PRESSURE,CONTINUOUSLY MEASURING THE RATE OF PRESSURE CHANGE AS THE ABSOLUTEPRESSURE INCREASES, CONTINUING TO INCREASE THIS FACTOR AS LONG AS NOINCREASE OCCURS IN SAID RATE REGARDLESS OF INCREASES IN THE ABSOLUTEPRESSURE UP TO THE LIMIT WHICH ITSELF CAUSES HANGING, AND CUTTING BACKTHIS FACTOR WHEN AN INCREASE IS DETECTED IN SAID RATE.